Add @grant for window.external

[kkmuffme]

Actual Behavior

Currently, a hardcoded list of URLs have access to window.external.Tampermonkey (#322)

Expected Behavior

It would be useful, if there was a way to @grant to allow access to them from the script on URLs this script runs on.
Possibly, using a custom object key to prevent detection, e.g.
@grant external.myTampermonkey

to be able to access window.external.myTampermonkey (instead of window.external.Tampermonkey)

Why?
It would often be helpful, if the script could open tampermonkey settings e.g.

window.external.Tampermonkey.openOptions( 'nav=dashboard' );

to guide users on set up or questions (since the chrome-extension:// URLs won't work with window.open)

Additionally, in a corporate setting, it's often much faster to check which tampermonkey scripts are installed/their version, than having to wait for someone to update any group policies,...

Specifications

• TM: 5.3.3